Endodontic instruments may be used for cleaning and enlarging the endodontic cavity space (“ECS”), also known as the root canal system of a human tooth. The unprepared root canal is usually a narrow channel that runs through the central portion of the root of the tooth. Cleaning and enlargement of the ECS may be necessitated by the death or necrosis of the dental pulp, which is the tissue that occupies that space in a healthy tooth. This tissue may degenerate for a multitude of reasons, which include tooth decay, deep dental restorations, complete and incomplete dental fractures, traumatic injuries or spontaneous necrosis due to the calcification and ischemia of the tissue, which usually accompanies the ageing process. Similar to a necrotic or gangrenous appendix, the complete removal of this tissue is paramount, if not urgent, because of the subsequent development of infections or dental abscesses, septicemia, and/or otherwise.
The root canal system of a human tooth is often narrow, curved and calcified, and may be extremely difficult to negotiate or clean. Indeed, the conventional endodontic or root canal instruments currently available are frequently inadequate in the complete removal of the pulp and the efficient enlargement of the ECS. Furthermore, they are usually predisposed to breakage, causing further destruction to the tooth. Broken instruments are usually difficult, if not impossible to remove, often necessitating the removal of the tooth. Injury to the tooth, which occurs as the result of a frank perforation or alteration of the natural anatomy of the ECS, may also lead to failure of the root canal and tooth loss.
The unprepared root canal of the tooth usually begins as a narrow and relatively parallel channel. The portal of entry or the orifice and the portal of exit or foramen are relatively equal in diameter. To accommodate complete cleaning and filling of the canal and to prevent further infection, the canal must usually be prepared. The endodontic cavity preparation (“ECP”) generally includes progressively enlarging the orifice and the body of the canal, while leaving the foramen relatively small. The result is usually a continuous cone-shaped preparation.
In general, endodontic instruments are used to prepare the endodontic cavity space as described above. Endodontic instruments may include hand instruments and engine driven instruments. The latter may but need not be a rotary instrument. Combinations of both conventional hand and engine-driven rotary instruments are usually required to perform an ECP successfully and safely.
An endodontic instrument includes a shaft that includes a tip and a shank. The endodontic instrument also includes grooves that generally spiral around the shaft. The grooves are referred to in the instant specification as flutes as shown in FIG. 1 and FIG. 2, which is a cross-section of the endodontic instrument of FIG. 1.
With reference to FIGS. 1-2, an endodontic instrument (e.g., endodontic file), generally denoted as 10, has a shaft 12 tapered along at least a portion of its length 15 and terminating at a point 14. A portion of the shank above the tapered portion is illustrated as being substantially cylindrical. Helical flutes 16 are formed in the tapered portion 15 of the shaft 12 and define helical cutting edges 20.
The flutes are generally the spacing on both sides of a helical structure (or helix) that spirals around the shaft. The bottom portion of a flute—seen as a line or curve is referred to in the instant specification as a spline 22. The portion of a spline that comes into contact with the surface being cut during cutting will be referred to in the instant specification as a radial land 24. Generally, an instrument having right-handed cutting edges is one that will cut or remove material when rotated clockwise, as viewed from shank to tip 14. In this specification, a direction of rotation will be specified as viewed from the shank to the tip of the instrument. The cut direction of rotation for a right-handed endodontic instrument is clockwise. An instrument having left-handed cutting edges is one that will cut or remove material when rotated counter-clockwise. The cut direction of rotation, in this case, is counter-clockwise. An instrument may also reciprocate, or move forward and reverse and have either a right handed or left handed flute. In general, a reciprocating endodontic instrument will move in one direction further than the other with the handedness of the endodontic instrument being associated with the larger angle of rotation.
An endodontic instrument includes a working portion 26, which is the portion that may cut or remove material. The working portion is typically the portion along the shaft that is between the tip 14 of the instrument and the proximal end portion 28 of the flutes. The working portion is also referred to in this specification as the cutting portion, and the working length as the cutting or working length.
Hand instruments are typically manufactured from metal wire blanks of varying sizes. The metallurgical properties of these wires, in general, have been engineered to produce a wide range of physical properties. These wires are usually then twisted or cut to produce specific shapes and styles. Examples of hand instruments include K-type, H-type, and R-type hand instruments. The barbed broach is manufactured from soft iron wire that is tapered and notched to form barbs or rasps along its surface. These instruments are generally used in the gross removal of pulp tissue or debris from the root canal system. Another R-type file is a rat-tail file.
K-type instruments in current usage include reamers and K-files. K files are generally available in carbon steel, stainless steel, and more recently, an alloy of nickel-titanium. To fabricate a K-type instrument, a round wire of varying diameters is usually grounded into three or four-sided pyramidal blanks and then rotated or twisted into the appropriate shapes. These shapes are specified and controlled by the American National Standards Institute (“ANSI”) and the International Standards Organization (“ISO”). The manufacturing processes for reamers and files are similar; except however, files usually have a greater number of flutes per unit length than reamers. Reamers are used in a rotational direction only, whereas files may be used in a rotational or push-pull fashion. Files made from three-sided or triangular blanks have smaller cross sectional areas than files made from four-sided blanks. Thus, these instruments are usually more flexible and less likely to fracture. They also may display larger clearance angles and are more efficient during debridement. Triangular files, therefore, are generally considered more desirable for hand instrumentation.
H-type files are usually manufactured by grinding flutes into tapered round metal blanks to form a series of intersecting cones. H-type files may usually cut only in the pull direction (i.e., a pull stroke). Primarily because they have positive cutting angles, H-type files may be extremely efficient cutting instruments.
Hand instruments are usually manufactured according to guidelines of the ANSI and the ISO, which specified that a working portion of an instrument be 16 mm in length. ANSI and ISO further specified that a first diameter or D.sub.1 of the instrument, be 1 mm from the tip or D.sub.0. Other ANSI and ISO specifications require that: instruments have a standard taper of 0.02 mm per mm along the working portion; the tip maintain a pyramidal shape no greater than 75 degree in linear cross section; and hand instruments are available in 21, 25, and 31 mm lengths.
In addition to the hand instruments described above, there are rotary instruments that are usually motor driven. G-type drills are usually available in carbon or stainless steel. As is typical, the G-type drill 300 shown includes a short flame-shaped head attached to a long shank. The flutes, in this instance, have U-shaped splines. The instrument includes cutting edges that have negative rake-angles. In general, a rake angle is the angle between the leading edge of a cutting tool and a perpendicular to the surface being cut. Rake angle is further described below. The flame-shaped head includes a non-cutting surface to prevent perforation. The instrument may be used as a side-cutting instrument only. The instrument is relatively rigid and, therefore, cannot usually be used in a curved space, for example, the ECS.